Enantioselective synthesis of a 3'-dephenylcryptophycin synthon

An enantioselective synthesis of tert-butyl (5S,6R)-(E)-5-tert-butyldimethylsilyloxy-6-methyl-2,7-octadieno ate, a precursor for the synthesis of the antimitotic macrolides cryptophycin A and arenastatin A (cryptophycin-24), is presented. The key step in the reaction sequence features a crotyl boration that sets both stereocenters that become the C16 hydroxyl and Cl' methyl in the cryptophycins. Homologation of the terminal olefin via a Heck reaction is presented.